Krypton and xenon are present in air as trace components (1.14 ppm and 0.086 ppm, respectively) and can be produced in pure form from the cryogenic distillation of air. Both of these elements are less volatile (higher boiling) than oxygen and therefore concentrate in the liquid oxygen sump in the low pressure column in a conventional double column air separation unit. Impurities that are less volatile than oxygen, such as methane, will also concentrate in the liquid oxygen sump along with krypton and xenon. Unfortunately, process streams containing oxygen, methane, krypton and xenon present a safety problem due to the combined presence of methane and oxygen.
Methane and oxygen form flammable mixtures with a lower flammability limit of 5% methane in oxygen. In order to operate safely, the methane concentration in an oxygen stream must not be allowed to reach the lower flammability limit and, in practice, a maximum allowable methane concentration is set that is a fraction of the lower flammability limit. This maximum effectively limits the concentration of the krypton and xenon that are attainable as any further concentration of these products would also result in a methane concentration exceeding the maximum allowed. Therefore, it is desirable to remove methane from the process.
Methane is currently removed from the krypton and xenon concentrate stream using a burner that operates at 800.degree.-1000.degree. F. The burning of methane produces two undesirable by-products, water and carbon dioxide, in the process stream. These impurities are typically removed by molecular adsorption. Therefore, the current method of removing methane requires a methane burner, an adsorption system, and several heat exchangers to warm the stream from a cryogenic temperature to the burner temperature and then back to a cryogenic temperature after the adsorption step. Methane removal in this manner also results in some loss of krypton and xenon.
Numerous processes are taught in the background art, among these are the following:
U.S. Pat. No. 4,647,299 discloses a process that concentrates krypton and xenon in a liquid product stream from a feed containing oxygen, krypton, xenon, and methane. The objective of this process is to alleviate the safety concerns associated with streams containing oxygen and methane by removing oxygen. Oxygen removal is accomplished in a single distillation column. In the oxygen removal, a feed liquid, containing oxygen, krypton, xenon, and methane is fed into a distillation column at an intermediate point as shown in FIG. 1. A vapor stream, containing less than 2% oxygen, is introduced to said column at a point below said intermediate point. A liquid, containing less than 3 ppm krypton and less than 0.2 ppm xenon is introduced above said intermediate point to provide reflux. Additional vapor is provided by reboiling downflowing liquids in a reboiler located at the bottom of said column. A liquid product stream, concentrated in krypton and xenon and substantially oxygen-free is withdrawn from the bottom of said column.
In the example presented in U.S. Pat. No. 4,647,299 the vapor feed to the bottom of the column was gaseous nitrogen and the reflux liquid fed to the top of the column was liquid nitrogen. The gaseous nitrogen introduced below the feed point strips downflowing liquid of oxygen such that liquid product withdrawn from the bottom of the column contains 0.8% oxygen and 97.1 nitrogen. The concentration of krypton and xenon increased from 443 ppm and 38 ppm, respectively, in the feed, to 15000 ppm krypton and 2000 ppm xenon in the liquid product stream. However, the hydrocarbon concentration of about 4000 ppm in the liquid product stream was the same as in the intermediate feed stream. The process described in U.S. Pat. No. 4,647,299 alleviates the problems involved with methane/oxygen mixtures by removing oxygen from the process. Most of the hydrocarbons are not removed in this cryogenic distillation and must be removed by further processing of the liquid product stream.
Another process that addressed the safety concerns (associated with oxygen-methane mixtures) in the production of krypton and xenon was disclosed in U.S. Pat. No. 3,596,471. In this process, liquid oxygen withdrawn from the low pressure column sump is fed to an adsorber that removes hydrocarbons, with the exception of methane, and then to the top of an oxygen stripping column. Vapor in the column is provided by a gaseous argon stream fed at the bottom of the column. The rising vapor strips the descending liquid of oxygen and is recycled to the argon column. Liquid product withdrawn from the sump of the oxygen stripping column contains oxygen, krypton, xenon and methane in argon. Introduction of argon into the bottom of the oxygen stripping column effectively displaces oxygen such that the product stream does not contain enough oxygen to form a flammable mixture with methane. However, methane and residual oxygen in the product stream must be removed prior to obtaining pure krypton and xenon. Methane is removed in a methane burner and residual oxygen is removed in a second distillation column. The patent also discloses a process illustrated in East German Pat. No. 39707 in which oxygen is stripped with gaseous nitrogen (instead of argon). The patent teaches that "due to equilibrium conditions, the replacement of oxygen by nitrogen remains incomplete, and the result is poor rectification in the stripping column."
U.S. Pat. No. 3,596,471 also discusses two West German Pat. Nos. 1,099,564 and 1,122,561 where attempts were made to remove methane rather than oxygen. The processes of these patents used extensive vaporization of liquid oxygen due to the dilution of the hydrocarbons by adsorption, however, methane cannot be entirely eliminated by this method.
Another process that produces a stream concentrated in krypton and xenon by cryogenic methods is disclosed in U.S. Pat. No. 4,401,448. The process uses two columns to concentrate krypton and xenon in addition to the standard double column ASU. In this process, a gaseous oxygen (gaseous oxygen) stream is withdrawn from below the first tray of the low pressure column and fed below the first tray of the rare gas stripping column. Reflux for this column is provided by a liquid oxygen stream withdrawn from the low pressure column at a point above where the gaseous oxygen stream was taken. Boilup in the rare gas stripping column is provided by indirect heat exchange with a gaseous nitrogen stream from the HP column. Vapor exiting from the top of the rare gas stripping column operates at a reflux ratio of 0.1 to 0.3 (preferred value 0.2). Liquid that is concentrated in krypton, xenon and hydrocarbons is withdrawn from the bottom of rare gas stripping column is fed to the top of the oxygen exchange column. A gaseous nitrogen stream, taken from the HP column, is introduced below the first stage of the oxygen exchange column such that the reflux ratio is 0.15 to 0.35 (preferred value 0.24). Boilup in the oxygen exchange column is provided by indirect heat exchange with a gaseous nitrogen stream from the HP column. Vapor exiting the top of the oxygen exchange column is recycled to the low pressure column. A liquid product that is concentrated in krypton and xenon is withdrawn from the bottom of the oxygen exchange column.
U.S. Pat. No. 4,4O1,448 reports results from a computer simulation of the process described above. The liquid product stream withdrawn from the oxygen exchange column contained 1.0% oxygen, 11000 ppm krypton, 900 ppm xenon, and 3200 ppm hydrocarbons with balance being nitrogen. This scheme alleviated two problems associated with prior processes. First, introduction of nitrogen at the bottom of the oxygen exchange column effectively displaces oxygen such that the product stream withdrawn from this column does not contain enough oxygen to form a flammable mixture with hydrocarbons. Second, the process is cryogenic. Krypton recovery was calculated as 72% from data presented in the patent and such a low recovery is undesirable.